Commit 9181a980 authored by David Hildenbrand's avatar David Hildenbrand Committed by Linus Torvalds
Browse files

mm: document semantics of ZONE_MOVABLE 



Let's document what ZONE_MOVABLE means, how it's used, and which special
cases we have regarding unmovable pages (memory offlining vs.  migration /
allocations).

Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Acked-by: default avatarMike Rapoport <rppt@linux.ibm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Qian Cai <cai@lca.pw>
Link: http://lkml.kernel.org/r/20200816125333.7434-7-david@redhat.com


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 27f85279

include/linux/mmzone.h

+35 −0
Original line number Diff line number Diff line
@@ -396,6 +396,41 @@ enum zone_type { 
	 */

	ZONE_HIGHMEM,

#endif

	/*

	 * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it contains

	 * movable pages with few exceptional cases described below. Main use

	 * cases for ZONE_MOVABLE are to make memory offlining/unplug more

	 * likely to succeed, and to locally limit unmovable allocations - e.g.,

	 * to increase the number of THP/huge pages. Notable special cases are:

	 *

	 * 1. Pinned pages: (long-term) pinning of movable pages might

	 *    essentially turn such pages unmovable. Memory offlining might

	 *    retry a long time.

	 * 2. memblock allocations: kernelcore/movablecore setups might create

	 *    situations where ZONE_MOVABLE contains unmovable allocations

	 *    after boot. Memory offlining and allocations fail early.

	 * 3. Memory holes: kernelcore/movablecore setups might create very rare

	 *    situations where ZONE_MOVABLE contains memory holes after boot,

	 *    for example, if we have sections that are only partially

	 *    populated. Memory offlining and allocations fail early.

	 * 4. PG_hwpoison pages: while poisoned pages can be skipped during

	 *    memory offlining, such pages cannot be allocated.

	 * 5. Unmovable PG_offline pages: in paravirtualized environments,

	 *    hotplugged memory blocks might only partially be managed by the

	 *    buddy (e.g., via XEN-balloon, Hyper-V balloon, virtio-mem). The

	 *    parts not manged by the buddy are unmovable PG_offline pages. In

	 *    some cases (virtio-mem), such pages can be skipped during

	 *    memory offlining, however, cannot be moved/allocated. These

	 *    techniques might use alloc_contig_range() to hide previously

	 *    exposed pages from the buddy again (e.g., to implement some sort

	 *    of memory unplug in virtio-mem).

	 *

	 * In general, no unmovable allocations that degrade memory offlining

	 * should end up in ZONE_MOVABLE. Allocators (like alloc_contig_range())

	 * have to expect that migrating pages in ZONE_MOVABLE can fail (even

	 * if has_unmovable_pages() states that there are no unmovable pages,

	 * there can be false negatives).

	 */

	ZONE_MOVABLE,

#ifdef CONFIG_ZONE_DEVICE

	ZONE_DEVICE,